It is proposed to investigate the enzymatic mechanism of postreplication repair and general genetic recombination, controlled by the recA gene of E. coli. Genetic exchanges appear to be initiated between duplix molecules when one of them contains single stranded regions such as postreplication gaps or incised crossinks, lesions that are not rapidly repaired. We plan to investigate the binding of RecA protein and single strand binding protein from E. coli to intact duplex DNA molecules that are covalent circular or blunt ended linear, or are duplex molecules that contain short single stranded regions (gapped molecules). The nature of the complexes so formed will be examined for their ATPase activity, normally indicative of the RecA protein being bound to single stranded DNA, and for their structure by various physical methods. We will investigate the binding between intact and gapped circular DNA promoted by these proteins, testing for the formation of stable homology dependent complexes. We will test the effect of substituting at the termini at the single stranded regions. We will examine the nature of the pairing between intact and gapped duplex molecules and try to distinguish whether homologous pairing proceeds through the formation of D-loops with Watson-Crick pairing or four strand synapsis with McGavin (1971) pairing. We will try to find other proteins needed for making recombinants, testing the intact molecules for cutting in trans, and will attempt to develop an in vitro system for postreplication repair.